This invention relates to a method for checking a torque sensor used in an electric power steering system for a motor vehicle.
There has been known an electric power steering system for a motor vehicle in which a current flowing through an assist motor is controlled according to a torsion torque applied to a steering shaft, and the steering operation is assisted by the assist motor. For instance, in copending patent application now laid-open Patent No. 98670/1986, there is disclosed an electric power steering system wherein an assist signal is generated in response to the output signal of a torsion torque sensor while a return signal is generated in response to the output of a steering angle sensor. The return signal multiplied by a constant is added to the assist signal for reducing the output torque of the steering motor.
In the conventional electric power steering system described above, troubles by an excessive torque signal or an extremely small torque signal may occur in the torque sensor. The excessive torque signal increases the power assist, causing over steering, while the extremely small torque signal entails so-called heavy steering. The former trouble of the torque sensor can be easily detected by a frequent check whether the magnitude of the signal is in an allowable range or not. However, the detection of the later-mentioned trouble is not easy, particularly when the torque signal is near zero, because such a trouble cannot be discriminated from normal conditions that the steering wheel is not rotated so that no torque is applied to the steering shaft. There has been no simple detecting method of this kind of trouble, and therefore presumption was a common practice to find out this kind of trouble from the heavy steering condition described above.